Image sensor device comprising protection against overflow

ABSTRACT

The invention relates to an image sensor device comprising a substrate, formed in CMOS technology, in particular, with an integrated semiconductor structure (ASIC) and, arranged above that, an optically active thin-film structure comprising in each case at least one layer made of doped and undoped amorphous silicon, spatially adjacent pixels in each case being formed in the horizontal plane, which pixels each have an optoelectronic transducer for converting incident light into an electric current proportional to the incident quantity of light and also a charge store assigned to the optoelectronic transducer, the charge state of which charge store can be varied in a manner dependent on the light incident on the assigned optoelectronic transducer. The invention is based on the object of avoiding an image corruption in the individual pixel on account of a transition of the optoelectronic transducer from the off state, which object is achieved according to the invention by virtue of the fact that the charge store is assigned first and second limiting means for defining in each case an upper and a lower charge limit value.  
     The single figure in designated for the publication.

[0001] The invention relates to an image sensor device comprising a substrate, formed in CMOS technology, in particular, with an integrated semiconductor structure (ASIC) and, arranged above that, an optically active thin-film structure comprising in each case at least one layer made of doped and at least one made of undoped amorphous silicon, spatially adjacent pixels in each case being formed in the horizontal plane of the image sensor device, which pixels each have an optoelectronic transducer for converting incident light into an electrical signal proportional to the incident quantity of light, in particular an electric current, and also a charge store assigned to the optoelectronic transducer, the charge state of which charge store can be varied in a manner dependent on the light incident on the assigned optoelectronic transducer.

[0002] An image sensor device of the type mentioned is known from the art. Such an image sensor comprises a substrate, usually fabricated in CMOS technology, with semiconducting properties and a structure of optically active layers deposited thereon. This technology is also referred to as thin film on ASIC (TFA technology). In image sensors of this type, the optically active layers are situated on the top side of the basic substrate structure, and individual pixels are in each case formed there in a horizontally adjacent manner. Each individual pixel contains an optoelectronic transducer in the form of a photodiode in which the incident light is converted into an electrical signal proportional to the incident quantity of light, in particular an electric current. The quantity of light is detected by means of a charge store assigned to the optoelectrical transducer, the charge state of which charge store is a measure of the quantity of light that has been incident within a specific time duration.

[0003] After the measurement time, the so-called integration duration, has elapsed, the measured value from the individual pixel which is present in the charge store is controlled, read out and processed further by peripheral electronics.

[0004] One property of such known image sensors is that the use of photodiodes as optoelectronic transducers of the optical sensor has the following disadvantageous effect:

[0005] If the photodiode, usually operated in the off state, undergoes transition to the open circuit or to the on state, the photodiode loses its desired effect and an interaction with the photodiodes of the adjacent pixels takes place, so that an image corruption occurs as a result of this. This leads to a white spot on the image sensor in the region of the disturbed pixel. This effect is referred to as so-called “blooming”.

[0006] Taking this as a departure point, the invention is based on the object of avoiding an image corruption in the individual pixel on account of a transition of the optoelectronic transducer from the off state.

[0007] This object is achieved according to the invention by virtue of the fact that the charge store is assigned first and second limiting means for defining in each case an upper and a lower charge limit value.

[0008] The invention is distinguished by the fact that, on the one hand, the definition of the upper limit value for the charge store means that an initial state is defined for the integration of the pixel-related measured value, and that, on the other hand, the definition of the lower limit value means that the charge store always has a minimum residual potential which prevents the optoelectronic transducer (i.e. the photodiode) from undergoing transition to the open circuit or the on state. The voltage applied to the charge store for this purpose is at least high enough that the semiconductor switch already undergoes transition to the on state before the voltage of 0 volts is reached at the charge store. This ensures that the voltage across the charge store and thus the voltage across the detector cannot fall below a specific value. As a result of this the detector can no longer attain the open circuit or the on state, thus preventing the disadvantageous “blooming” effect.

[0009] Preferred embodiments can be gathered from the subclaims.

[0010] The invention is explained in more detail below on the basis of an exemplary embodiment (figure):

[0011] The electronic circuit illustrated in the figure relates to an individual pixel and comprises an optoelectronic transducer which is embodied as a detector and with which a charge store in the form of a capacitor C_(int) is connected in parallel. One of the common terminals is connected to the ground potential G_(nd), and the other common terminal of detector and capacitor is connected to the collector terminal of a semiconductor switch T_(res), whose emitter terminal is connected to a fixed reference voltage V_(res) and whose base terminal is connected to a base input RESET. The remaining elements illustrated in the circuit relate to the operation of reading out the measured value stored in the charge store and are not of importance for the understanding of the present invention.

[0012] At the beginning of the measurement operation, a sufficiently high first voltage is applied to the reset input of the semiconductor switch, which voltage causes the semiconductor switch to turn on. The voltage V_(res) is thereby applied to that terminal of detector and capacitor C_(int) which is not at ground potential. This voltage defines the upper charge state of the capacitor C_(int). Consequently, the voltage V_(res) is present as reverse voltage at the detector.

[0013] The base voltage of the semiconductor switch is subsequently reduced to a value which, on the one hand, still lies a little above the reverse voltage of the semiconductor switch but, on the other hand, is so low that the semiconductor switch undergoes transition from the on to the off state. In the following operating phase of the optical sensor, the capacitance C_(int) is discharged by the detector current of the optoelectronic transducer, to be precise until the voltage in the capacitor has fallen to a value of a little above 0 volts. However, before the voltage of 0 volts is reached at the upper terminal of the capacitor C_(int), the semiconductor switch again undergoes transition to the on state, since the voltage present at the control input is greater than the pure reverse voltage. This ensures that a reverse voltage having a minimum magnitude is always present at the detector, so that the latter cannot attain the open circuit or the on state. 

1. An image sensor device comprising a substrate, formed in CMOS technology, in particular, with an integrated semiconductor structure (ASIC) and, arranged above that, an optically active thin-film structure comprising in each case at least one layer made of doped and at least one made of undoped amorphous silicon, spatially adjacent pixels in each case being formed in the horizontal plane of the image sensor device, which pixels each have an optoelectronic transducer for converting incident light into an electrical signal proportional to the incident quantity of light, in particular an electric current, and also a charge store assigned to the optoelectronic transducer, the charge state of which charge store can be varied in a manner dependent on the light incident on the assigned optoelectronic transducer, characterized in that the charge store is assigned first and second limiting means for defining in each case an upper and a lower charge limit value.
 2. The image sensor device as claimed in claim 1, characterized in that the charge store is a capacitor (C_(int)).
 3. The image sensor device as claimed in claim 1, characterized in that the first limiting means is a reset means by which a predeterminable electrical voltage V_(res) can be applied to the charge store for the purpose of defining the upper charge limit value.
 4. The image sensor device as claimed in claim 1, characterized in that the second limiting means is a semiconductor switch whose control terminal is held at a potential so little above the reverse voltage that the semiconductor switch undergoes transition to the on state before the potential at the capacitor reaches a critical value, in particular the value 0 volts.
 5. The image sensor device as claimed in claim 1, characterized in that the image sensor device has a row of individual pixels.
 6. The image sensor device as claimed in claim 1, characterized in that the image sensor device has an areal arrangement of pixels.
 7. The image sensor device as claimed in claim 1, characterized in that the thin-film structure contains one or more thin layers of doped and/or undoped amorphous silicon.
 8. The image sensor device as claimed in claim 1, characterized in that the thin-film structure contains one or more thin layers of metal.
 9. The image sensor device as claimed in claim 1, characterized in that the thin-film structure contains one or more thin layers made of a transparent oxide. 